1. Field of the Invention
This invention relates to the general field of chemical agents which control insects pests, especially repellent agents which discourage the presence of such pests, rather than insecticidal agents which kill them.
2. Description of the Prior Art
In many instances it is more advantageous to repel insect pests from a given place than it is to kill them after they have entered it. Some insect pests inherently are difficult to kill, or the nature of the place where the pests are found may make it undesirable to kill them there. It is considered to be more desirable to repel pests such as cockroaches and flies from the vicinity of food, food handling equipment and places where food is eaten, since contacts between food and transitory insects serve to spread disease. In a number of instances, such as in the treatment of growing crops, it is desirable to use repellents to prevent the movement of insect pests into an area and from one area to another. Foliage-feeding insect pests may be killed by applying an insecticide to the leaves upon which they feed, but death of the pests after they have fed on the foliage is less than satisfactory in the case of many crops; for example, the market value of tobacco and cabbage is decreased markedly by holes in the leaves caused by foliage-feeding insects.
Of course, if the repellent treatment must be renewed too often, requiring time-consuming reapplication of an expensive chemical, the fundamental advantages of repelling rather than killing the insects are outweighed by economic considerations. Thus, insect repellents with long residual activity are especially desired.
Over the years a number of different oils, greases, ointments and powders have been employed as insect repellents with varying degrees of success. Oil of citronella was reported to be an effective insect repellent as early as 1901. Another natural product, nicotine from tobacco, was used as a repellent as long ago as 1760. Since World War II, a number of synthetic insect repellents have been introduced. These include 2-hydroxyethyl octyl sulfide (U.S. Pat. No. 2,863,799), used principally as a cockroach repellent; organic diols (U.S. Pat. No. 2,407,205); N,N-diethylbenzamides (U.S. Pat. No. 2,408,389); esters of pyridine dicarboxylic acids (U.S. Pat. No. 2,757,120); amido detergent alkylates (U.S. Pat. No. 3,234,081); and 3,3-dibutyl-2-(2,2,4-trimethylpentyl) pseudourea acetate (U.S. Pat. No. 3,266,978). None of these is noted for especially long residual activity.
Pyrethrins, which are cyclopropanecarboxylates, the naturally occurring extracts from the blossoms of pyrethrum flowers (Chrysanthemum cinerariae-folium) grown mainly in East Africa, were widely used as insecticides before the advent of synthetic materials such as DDT. The pyrethrins are effective in killing a wide range of insect species, and they also function as insect repellents [Burden, Pest Control, 43, 16 (1975)], but they are short-lived. Although they display relatively low toxicity toward mammals and do not leave harmful residues, they undergo rapid biodegradation, they have poor photooxidative stability, their availability is uncertain, and it is costly to extract and process them. Thus, their use has been limited. For a number of years, efforts have been underway around the world to produce synthetic cyclopropanecarboxylate insecticides, pyrethroids, which would overcome these disadvantages. 5-Benzyl-3-furylmethyl chrysanthemate (resmethrin) [Elliott, et al., Nature, 213, 493 (1967)], a powerful contact insecticide, was an early success. A notable recent result of these efforts was the discovery of the pyrethroid, 3-phenoxybenzyl 3-(.beta.,.beta.-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate, having the combination of toxicity and knockdown attributes of the natural pyrethrins and resmethrin together with a previously unattained level of photooxidative stability [Elliott, et al., Nature, 246, 169 (1973); S. African Patent 73/3528].
Although the activity of 3-phenoxybenzyl 3-(.beta.,.beta.-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate as an insect killer is disclosed in the prior art last cited, there has been no suggestion that this pyrethroid would exhibit the variant of insect control which is the subject of this invention - repellency.